1. Field of the Invention
The present invention relates to differential pressure operated servo-boosters, especially for vehicle brake systems, and more particularly to improvements in servo-boosters of the type in which the interior of a booster shell is divided by a booster piston into a forward first fluid chamber normally in communication with a vacuum source and a rearward second fluid chamber, and a cylindrical valve body integrally extended at the rearward surface of the booster piston is slidably supported on the rearward wall of the booster shell through a plain bearing, the valve body receiving therein an input rod and a control valve actuated by the input rod to permit the second fluid chamber to selectively communicate with the first fluid chamber or atmosphere so that when atmospheric pressure is introduced into the second fluid chamber, the booster piston is actuated by the pressure difference between the first and second fluid chambers.
2. Description of the Prior Art
In a conventional servo-booster of this type, as shown in FIG. 2, a plain bearing 010 carrying a cylindrical valve body 08 is fitted into a cylindrical support portion 03a extended from the rearward side wall of a booster shell 01 and retained in position by means of an elastic stop ring C, and accordingly, the number of parts and machining portions increases. It is therefore extremely difficult in terms of machining accuracy to register three elements, the booster shell 01, the plain bearing 010 and the valve body 08 on the same axis. In addition, with respect to the plain bearing 010, there is required fluid tightness at two portions, one being of the fitted portion between the bearing and the cylindrical support portion 03a, the other being of the sliding portion between the bearing and the valve body 08. This requires an additional care in machining and assembling the device, resulting in an obstacle in an attempt to reduce the cost of the device.